Tunable magnetron



8- s. SONKIN 2, 1

TUNABLE MAGNETRON Filed. Oct. 19, 1945 2 Sheets-Sheet 1 I FIG. 1.

INVENTOR. SIMON SONKIN ATTO R N EY Oct. 5, 1948. I s. SONKIN 2,450,619

I TUNABLE MAGNETRON Filed Oct. 19, 1945 2 Sheets-Sheet 2 FIG. 2.

3 5 0 r '5 3 v CL 5 o 050 m 20 Z Q g l- O .|oo E g IO w UJ (L I].

. -.ISO 1 l l 3.1 3.2 3.3 I 3.4 3.5 3.6

WAVE LENGTH (CM) v I INVENTOR. FIG.3. SIMON SONKIN ZMMQ ATTOR N EYPatented Oct. 5, 1948 2,450,619- TTJNABEE MAGNETRON' Simon Sonkin, NewYork, N. Y., assignor to the United States of America as represented bythe Secretary of War Application October 19, 1945, Serial No. 623,424 '1Claim. (01. nip-27.5)

The invention described herein may be manufa'ctured and'used by or forthe Government for governmental purposes, without the payment to me ofany royalty thereon.

This invention relates to ultra-high frequency generators of magnetrontype in which ultrahigh frequency oscillations are generated by aplurality of resonators set into oscillations by high velocity electronsmoving along curvilinear orbital" paths, these paths being followed bythe electrons because of the joint action of the electrostatic andelectromagnetic fields.

It is an object of this invention to provide positive means foradjusting the frequency of oscillation of a magnetron.

An additional object of this invention is to provide a tunable magnetronin which tuning is accomplished by means of a plurality of metallictuning pins which protrude into one of the end spaces of the magnetronthrough the adjacent pole piece, the mechanism for inserting the pinsinto the cavities of the anode being mounted in a well within the polepiece.

Still another object of this invention is to provide a tunable magnetronhaving a reluctance of its magnetic circuit equal to, or if so desiredsmaller than an identical untunable magnetron.

Yet another object of this invention is to provide a tunable magnetroncapable of covering a reasonably large tuning range.

The additional objects of this invention are the provision of tuninginstr'umentalities for inagnetrons which have reasonably long life of'all movable parts, reproducibility and stability of the desiredfrequency, and reasonable weight of the entire combination so that atunable magnetron does not weigh appreciably more than the fixedfrequency magnetron's.

The invention is an improvement of the tuning systems disclosed in myco-pendingapplications on tunable magnetron, Serial No. 623,422, filedUcto'ber I9, 1945, now abandoned, and Serial No; 623,423, filed October19, 1945, and especially on the tuning system disclosed in the firstapplication Where, in one embodiment of the invention, tuningofamagnet'ron' is accomplished by means of metallic pins insert'able into,the resonating cavities of an anode with the pin holding structureaswell as the pins being all mounted in one of the end spaces of themagnetron. Because of this type of mounting of the" tuning pins itbecomes necessary to increase the end space of the magnetron with theconcomitant increase in the reluctance of the magnetic circuit ofxth'e,tube s'oa that it the magnetic field is to remain conf Fig. 2.

2 stant with said structures as compared magnetic field of must be avery large increase in the coercive force to the supplied by thepermanent magnet. This can be accomplished only by increasing the sizeof the permanent magnets. When the weight requirements are critical,such increase in the weight of the permanent magnets increases the totalweight and bulk of the oscillator to undesirable proportions. Theadditional electrical disad-v vantage of such structure is in theincreased loading of the anode with the resulting lowering of itseificienoy. The mechanical disadvantage of the end space-mounted tuningsystem resides in the fact that it cannot be removed or inserted veryreadily into the pole piece since it represents an integrated structurewith-the pole piece. The invention avoids all of these difiiculties byplacing the tuning instrumentalities into one pole. piece with the pinsmoving through small holes in the pole piece and the supportingmechanism for the pins being mounted in a well within the pole piece.Using this superior relationship of the elements the use of the tuningpins does not produce an increase in the length of the magnetic gap sothat the reluctance of the magnetic circuit remains the same as in thenon-tunable magnetrons. g g ,7

These and other features of the invention will be more clearlyunderstood from the following detailed description and the accompanyingdrawings in which: l

Figure 1 is a vertical cross-sectional view of a tunable magnetron;

V Figure 2 is the cross-sectional view of the anode structure of themagnetron taken along line 22 illustrated in Fig. 1, and,

Figure 3 illustrates a series of typical performance curves obtainablewith the tuning arrangement disclosed in Figs. 1 and 2.

Referring to Figs l and 2-, they illustrate, by the way of an example, atunable, strapped, hole and-slot, planetary type magnetron having twelveresonating circuits, as illustrated more clearly in A mounting plate It,made of nonmagnetic metal such as bronze; supports the entire magnetronstructure. A glass boot I2 is ate tached to the mounting plate by studs."and I6, a fibre washer l8 being insertedbetween the glass boot and themounting plate. The lower portion of the glass boot is not illustratedin the dr-awing; it is constructed'in the known manner, terminatinginthe connections for the cathode and the heater circuits including aheaterbatteryil and a cathode-anodesource 15. A cathode pole the untunedmagnetron therepiece 20 is mounted on top of the mounting plate, thepole piece being spot-welded to the plate at i9 and 2!. The cathodestructure of the magnetron is axially mounted in a well 23 of thecathode pole piece, this structure including a (Kovar) eyelet 22 Weldedwith its flange 24 to the pole piece soas to form a hermetic sealbetween the eyelet and the pole-piece. The lower portion of the eyeletis connected to a re-entrant glass seal 26 which forms a gas sealbetween the (Kovar) tube 28 and the (Kovar) eyelet. The upper portion of(Kovar) tube 28 terminates in a cathode 30 which is provided with thecathode end-disks 32 and 34 which maintain the spacecharge conditionwithin the discharge space of the magnetron at desired values andelectrically seal off the discharge space from the .end zones of themagnetron. Within the (Kovar) tube 28 is mounted an insulated conductor36 the upper portion of which is connected to a heater coil 38 whichsupplies the necessary heat to the cathode. The outer surface of thecathode cylinder 39 is coated in well known manner with theelectronemissive oxides such as barium and strontium oxides. The cathodeis mounted centrally in the discharge space 40 of the magnetron and issup ported in this position by pole-piece 2D, the (Kovar) tube 28,eyelet 22 and glass seal 25. The anode 42 is made of a block ofconductive material such as copper shaped into a plurality of resonators 4| whose boundaries are defined by the cylindrical cavities 44and slots 45. The resonators terminate at the anode surfaces 68 whichsurround the central discharge space 49 of the magnetron. The inventionis disclosed with a l2-cavity anode but it is to be understood that theteachings of this invention are applicable to other types of anodeshaving a'smaller or larger number of cavities, and the cavities may haveeither a cylindrical shap illustrated in the figures or any otherdesired shape. When the shape of the cavities is modified the shape ofthe tuning pins should be modified also to follow the inductive surfacesof the cavities.

To separate the modes of oscillations of the resonators and tofacilitate the oscillations in the pi mode, the anode is preferablystrapped by means of straps 50 and ring type of strapping beingillustrated in the drawing. The anode of the magnetron is mounted in anon-magnetic metallic shell 54, this shell being welded to the uppertuning pole-piece 56 as well as the lower cathode pole-piece 20, asillustrated at 68, 69, 10, and II, respectively. The left portion ofshell 54 is provided with the cooling fins 55 while the right portion isprovided with an opening for connecting a coaxial line 58 whichterminates in a coupling loop 53 positioned inthe lower end zone of themagnetron directly under one of the holes in the anode. Only a portionof a flange 6B of the coaxial line is illustrated in the figure; theflange is connected to a wave guide by means of bolts which fit into theholes 62 of the flange. The coaxial line 58 is welded to shell 54 thusforming an air-tight joint between the non-magnetic copper stud 64 ofthe coaxial line and the shell. Only a portion of a glass seal 66 of thecoaxial line is visible in the drawing. The magnetic flux is provided bytwo permanent magnets fitting against the cathode and the tuningpole-pieces 20 and 56, only one magnet-magnet 51being visible in thefigure. The lowerportions of the magnets are fastened to plate [0, andthe'upper portions are inter-connected through flanges 59, BI and bolts63 and 65. The adjusting elements 4 of the tuning mechanism are mountedin wells 13 and 88 provided for this purpose in the upper pole-piece,which is called for this reason a tuning pole-piece.

The tuning mechanism includes twelve metallic tuning pins 12 protrudingthrough the pole-piece, an adjustable pin holder 14 made of magneticmaterial, a non-magnetic driving screw '16, Monel metal bellows 18, anon-magnetic tuning knob 80, a non-magnetic ball racer 82, andnon-magnetic balls 84. Wells 13 and 86 have their dimensionsproportioned so as to allow the desired degree of vertical travel of thepin holder and the concomitant vertical adjustment of the pins withinthe cylindricalcavities of the anode. The tuning elements areconstructed so as to permit complete withdrawal of the pins from theanode for maximum wave length, and lowering of the pins into the anodeuntil there is an evidence of excessive degree of coupling between theanode and the pins. This will be discussed more fully later in thespecification. The Monel metal bellows 1B are welded around their lowercircumference to the pole piece as illustrated at 88 and 89, and aroundtheir upper circumference to the upper flange of the pin'holder 14, at90 and 9|, thus completing the gas-tight'ioint between the pole pieceand the pin holder. The tuning pole piece is provided with twelve holesthrough which the tuning pins slide up and down during the adjustment ofthe frequency of the magnetron. The non-magnetic tuning knob iscalibrated in terms of wave length or frequency, the angular position ofthis knob'against a pointer 92, fastened to pole piece 56, indicatingthe frequency generated by themagnetron. Evacuation of the magnetron iseifected through a metallic tube 93 which is sealed upon the completionof the evacuation and degassing process of the magnetron.

Since the alignment of the tuning pins 72 with respect to the holes 44is accomplished by the ball bearing surfaces, it becomes necessary toconstruct the ball bearing so that there is no lateral play between theballs and the racers of the bearing. This is accomplished by providingsplit racers TI and 19, the lower racer 19 being held tightly againstthe balls 84 by a spring 8|. The spring itself is compressed against thelower, fiat surface of the racers" and 19 by means of a race holder 83.The race holder is fastened to the tuning knob 80 by aplurality of setscrews 85. The pressure exerted by spring 8| holds the balls of the ballbearing in a continuous engagement with all the racing surfaces of theracers so that any possibility of the lateral play in the ball bearingis thus eliminated. This construction of the ball bearing holds thedriving screw 16 in strict alignment with the common longitudinal centerline of the pole piece and the anode, and since screw 16 has a threadedengagement 6'! with pin holder '14, the pin holder itself, together withthe pins, is kept in the equally strict alignment with the holes in thetuning pole piece and the anode.

The tuning of the magnetron is accomplished by turning the tuning knob80. Since the nonmagnetic driving screw 16 is fastened to the tuningknob, it either raises or lowers the pin holder 14 because of thethreaded engagement 63 between the pin holder and the screw. The torqueexerted by screw 16 on the pin holder is resisted by the bellows 18which, as it will be remembered, are soldered on top to the pin holderand on the bottom to the pole piece. The tuning pins thus remaincentered in the holes of the pole piece ana mi and there isrnc twistinicrceexcrtednmn many-time. This isan importantieaturc when the, pins aremade .of pure copp 91s it Mm appear later in the specification.fltlhus-z turningi of knob 380 changes :the degreemf :pin :penetrationinto the cavities 44 wei the magnetron.

- Ehe nupper end :zone "1.5 of sthe .;;magnetr on is provided with: acopper: ring 29.4 which is proyided with twelve holes, the tuning pins.;passin through :the-:-ring. Ring .94 "performsa :very im- .-portantjunction --of shortening the electrical length of the pins; :thefunctioning ofiring Mcwill bedescribed later i-n thisispeciflcation.

The effect of the tuning pins on the resonators may lie-analyzed asfollows:

'The fundamentalmethod of varying the frequency of --any--resonatingcircuit is to change either the inductance -or 1 the capacity or both.Because the circuit constants inany planetary hole-and-slot magnetron-are-not lumped, it is of courseimpossible to-vary strict-1y only theinduc tanoeor only the capacity :of-any single resonating circuit.However, it is possibleto provide a tuning-meanswhich has apredominantly inductiveover-all eflect. In a magnetron oscillater oi thestrapped ho'le-and-slot type, it is diflicult'to introduce large changesin resonator capacity essentially because large capacity changes wouldinvolve the use of :very 'small clearances between the elementsat highradio frequency potential. Moreovenby "far the larger part of thecapacitive changes would take place at-the end portionof'the'mec'hanical movement,

that is,- when the clearances-between the tuning surfaces would beapproaching-zerovalues. Accordingly, it is a theoretical andcpracticalimpossibility to achieve any linear relationship'between the :Wavelength and-the movement of the tuning surfaces in'any capacitive tuningof the magnetron. The inductance of the resonating circuits may,however, be altered simply iby-the insertion of-any conductor into theinductive portion of the circuit, the conductoracting in effect as ashort-circuiting means for some portion of the inductance-present intheresonating circuit. Thus a large change in inductance may easily beintroduced into =amagnetron-of the hole-sandslot type by the simpleinsertion "of a cylindrical metallic pin into the cylindrical portion ofthe resonator with the pinsbeing nearest to the 'inductive portions'andfarthest from the capacitative portions of the resonators. Forthispurpose, each pin '12 is positioned-Withinthe hole of each resonatoralong the radial -axis195yFig. 2, thereof, 'but'farther from the slotentrance and nearer to the opposite surface of thehole. In the case'ofcircular'holes and pins, the-pins are eccentrically disposed within theholesas illustrated in Fig. '2 with the-pitch circle 96 of the holesbeing smaller than the pitch circle 91 of the-pins; stated-differently,the locus=91 oi the pin axesis a cylinder of greaterradius than thelocus 96 of the hole axes. Besides'obtaining maximum changes in I theinductance, such positioning of the tuning pins'also removes them fromthestamping ele ments 5!! and'5l thus avoiding any possibility ofaccidental-shorting of the resonating elements which'may take place "ifthe tuning pins happen totouch the strapping elements. This is animportant factor in the ultram-igh frequency =magnetrons having smallphysical dimensions.

"The number of tuning pins, as disclosed, is

equal to the number of cavities since it is desirableto obtain amaximumtuning-range with a givenfixed anode structure. There -=is also asci-meta thought.advibcatinewitneadcs rabilitacf altering-the inductance,of. ,all resonators gin -=a similar-way. r'nccordingtothisrl'fidSOfliIlg :thensc flf' i the tuningtpinfin' each-cylindricaljhcle would he' preferable since cltmiWOllld :insure 1 the \rmaintenanceof :an :angularly symmetrical fieldv pattern within the interactionvspace for all .'rinse .ntion depths .of the rzpins. :Gzeneralexperience, with mag-netrons indicates an :opti-mum rtuhe ,zoperam tionwhen such. symmetry -zexists.

fit lis W :possiblerto: analyze .theeiunction 11pm. termed r-rtheresonance suppressin ring :5. Gonsider 1a magnetron :with :the .geometryiridicatedin'PFigs; 1 iand?2. suchgeometry :thepins themselves, togetherwith the annular region between the pins andthee-anode shell, constitutean osciil-atorycircuit. 1 In :a general way the circuit-capacit-y occursbetween l the ,walls .-:andthe pins of the cylindrical resonator andthetindum tanoe in the-iannular; region. fI'hisisy-stem'of pins will :becharacterized. by-a= series of modes of cs1 .cillatiomeachcfa-characteristic wavelength. Whilethecharacteristic. wave length of the:anode decreases with -increasing .pin penetration, that of mach oi themodestof the pin system I will increase-with the increasing -.-.pinpenetration. 1 If for: some pin :penetration the .characteristicwavelengthsof r the anode :and of the pins structure are; coincident, and ifthere: is :anytcoupling between the two; large-deviation fromvnormalctube operation-may be expected. ZFirst; the resonant pinsystemniaypull the magnetron .wave length over :and above-Many alteration .inwave length lay-the lrnown resonant change in inductance; second, it mayabsorb significant fractio-n of the-electronically generated power;third, by-ithe distortion otthe radio frequency pattcrnsin themagnetron, it may impair the electronic cfii' c-iency-andgf-ourth, itmay greatly alter the pulling figure. Moreoven ther-emay be-a-pr-esencecf a very dominant auxiliary .effect in which the pin system12-apparently acts as 3, coupling device between ithe magnetron anode'dil'and the coaxial line 28-nwhich-supports the --cathode-which :diwarts-"a large -quantity of the ultra-high-irequency power lntothe-coaxial-line of the oathcda the latter phenomenon in an extreme casecompletely blocking the normal operation of' the magnetron.

In order that the tube may operate satisfactorily over-the entire tuning:range, it is therefore -neoessary*-that the -spectrum oithe pinstructure be -outside the range "of the operating wave lengths of the-magnet-ron. In general the -f-ree length of the pi-ns whic'helectrically eng-ages tlsie anode mustbe considerably less than Ml-since the magnetron end space contributes a -'large amount ofloading.It-is this function, that shorting of the --electrical "length of thetuning pins, that is performed by the resonance sup =pressing-ring 94rIt reduces to-aslargean extent as pessible the-luniped--inductance inthe end space and effectively shortens "the electrical length --of"thepins --en hancing-- the tuning properties of, the pins --and--theefflciency -of the Thus the -resonance-suppressing ring reduces to a-;minimum the-undesirable degree of coupling between theanodefresonators and the pins, the pirrresonanees beingmade bythe-resonance suppressing ring completely-beyond the-range of thetube-resonances.

= While-it may-be very desirable -and-even necessary-tense the-resonancesuppressing rings in-the end zones of the magnetrons-of somespecific wave 7:5 *length,-=the -'-physieal-relationships between theanode'g'the anode frequencies and the resonating properties of the'endspace inthe magnetrons of some other wave length'may be such that highoperating eiiiciency, frequency tracking, low pulling figure, etc.,'that is general stability of the system, may be obtained without the useof the resonance suppressing rings. Therefore the fact that theresonance suppressing ring may: prove to be an especially efifective anddesirable device in the case of the magnetrons of some specific wavelength'region, it does not necessarily follow that the ring hasa-general application and will produce the universal beneficial results.The governing factor in either accepting or rejecting the use of thering is the previously mentioned relationship between the operating wavelength (or some higher modes) of the magnetron and the electricalproperties of the end space and of the pins.

In describing the pin structure it has been mentioned that they are madeof conductive material. Since pins 12 introduce losses and reduce the Qvalue of the anode, it is important that the pins should have themaximum obtainable conductivity. From a purely mechanical point of viewit is desirable for the pins and the pin holder 14 to represent astructure with maximum possible rigidity, the rigidity of thisstructure; together with the back lash of the moving elements, primarilydetermining the resettability to a given Wave length from the dialreadings on knob 80. To satisfy the mechanical rigidity the pins may bemade of copper-plated tungsten. However, tungsten is very brittle anddifficult to work and, therefore, equally successful results may beobtained but with a lesser degree of mechanical effort if the pins aremade of copper-plated stainless steel; A measurement of losses in thepin material has indicated that the high temperature to which these pinsmust be subjected in assembly increases their resistivity, presumably bydifiusion of the components of the stainless steel into copper.Improvements in assembly technique eventually permitted the use of solidpure copper pins in spite of their tendency to distort under very smallforces; with the space and highest obtainable conductivity at a premiumin the disclosed tuning mechanism, pure copper is the commercial metalwith the best conductivity per unit of volume. 7

While the invention has been specifically disclosed in connection withthe hole-and-slot anode, it will be apparent to those skilledin the artthat the invention is applicable to the anode structures of differenttypes, such as vane type or Rising Sun type anodes, as disclosed in myco-pending application titled Tunable magnetron, Serial No. 623,422,filed October 19, 1945. The only requirement is that there is asufficient electrical coupling between the inductive portion of theresonatorand the pin so that the pin may act as a short-circuitingdevice for the distributed inductance of the resonator. The pins havinga cylindrical shape have been disclosed because it is obvious that thepins of this shape will be especially advantageous in short-circuitingtheinductance of the cylindrical holes. When the shape of the resonatingcavity is other than the cylindrical hole, the shape of the tuning pinmay be modified so as to engage the inductive portion of the resonatorin as efiective a manner as the round pin engages the round .hole. I Itis to be noted however that any sharpdiscontinuities in the-surfaceshould be avoided in order to prevent excessive losses'in the structureand accidental '8 flash-overs. The advantages obtained with thescalloped anodes as discussed in'the co-pending application SerialNo.623,422, are equally applicable to the principal case.

While the invention has been described with reference to severalparticular embodiments, it will be understood that various modificationsof the apparatus shown may be made within the scope of the followingclaims. l

1. An ultra-high frequency magnetron including an anode, a pole piecemounted in spaced relationship with respect to said anode, a pluralityof metallic members mounted within said pole piece, mechanicallyinstrumentalities for lowering said members from their normal positionwithin said pole pieceinto positions producting varying degrees ofcoupling between said members and said anode to vary the operatingfrequency of said anode, said pole piece having a well for housing saidinstrumentalities.

2. A tunable magnetron having an anode, a pole piece, said pole piecehaving a well, a holder mounted within said well, a plurality of tuningmembers attached to said holder, a corresponding plurality of openingsin said pole'piece, said tuning members fitting into said openings, andmeans joining said holder with an external controlling element forvarying the position of said holder within said well for varying thedegree of electrical engagement between the anode of said magnetron andsaid tuning members.

3. A magnetron having a slot-and-hole anode, a cathode centrally mountedwith respect to said anode, pole pieces mounted in spaced relationshipwith respect to said anode, and a plurality of round metallic pinsadjustably mounted within one of said pole pieces and protruding withtheir ends through said pole piece so as to be insertable into the holes'of said anode for' varying the operating frequency of said magnetron.

4. A magnetron as defined in claim 3 which further includesinstrumentalities for keeping the respective axes of said pins inalignment with the respective axes of said holes in said anode throughthe traveling range of said pins.

' 5. An ultra-high frequency tunable magnetron including a multicavityanode, a pole piece mounted in spaced relationship with respect to saidanode,-said pole piece defining an end space of said magnetron, saidpole piece having a well, a pin holder mounted within said well, aplurality of metallic-pins mechanically connected to said pin holder, 3,corresponding plurality of holes in said pole piece, said pinsprotruding through said holes into said end space, metallic bellowsinterconnecting said well and said pin holder, and a drive screwconnected to a rotatable knob and to said pin holder, said drive screwforming a threaded engagement with said pin holder whereby turning ofsaid knob raises or lowers said pins from or into the cavities of saidanode for varying the degree of electrical engagement between the anodeof said magnetron and said pins, the variation in said electricalengagement varying the operating frequency of said magnetron.

'6. An ultra-high frequency tunable magnetron including a cathode, amulticavity-multiresonator anode concentrically mounted with respect tosaid cathode, first and second pole pieces mounted in spacedrelationship with respect to said anode, a concentric line coaxiallymounted within said first pole piece, said line being connected to andsupporting said cathode, said second pole piece having a well, a holdermounted within said well, a plurality of metallic tuning membersfastened to said holder, a corresponding plurality of openings in saidsecond pole piece, said tuning members passing through said openings,metallic bellows interconnecting said well and said holder, a rotatableknob, a drive member interconnecting said knob and said holder, and aball bearing mounted on said second pole piece and having means forholding said tuning members in alignment with said cavities through saiddrive member and said holder, whereby turning of said knob in onedirection lowers said tunin members into the respective cavities of saidanode, and turning of said knob in the opposite direction withdraws saidtuning members from said cavities and draws them into said openings.

'7. An ultra high frequency tunable magnetron including anelectron-emissive cathode, an anode having a plurality of resonators andhaving at least one flat end surface, said cathode and said anode havinga common longitudinal axis; first and second pole pieces mounted inspaced relationship with respect to said anode, said pole piecesdefining first and second end spaces, respectively, above and below theend surfaces of said anode of said magnetron; said first pole piecehaving an axially positioned well, a pin holder mounted within saidwell, a plurality of metallic pins mechanically connected to said pinholder, said pole piece defining a corresponding plurality of holestherein, said pins protruding through said holes into said end space,metallic bellows hermetically sealed within said first pole pieceinterconnecting said well and said pin holder, and a drive screwconnected to an external controlling element and to said pin holder,said drive screw forming a threaded engagement with said pin holder, aball bearing mounted on said first pole piece and having means forholding members in alignment with said resonators through said drivemember and said holder, whereby turning of said controlling elementraises or lowers said pins from or into the cavities of said anode forvarying the degree of electrical coupling between the anode of saidmagnetron and said pins, the variation in said electrical couplingvarying the frequency of oscillations of said magnetron.

SIMON SONKIN.

REFERENCES CITED The following references are of record in the

